1. Field of the Invention
This invention relates to improvements in or relating to an anti-lock system for a motor vehicle, which is adapted to prevent the wheels of the motor vehicle from being locked during a braking operation.
2. Description of the Prior Art
There has conventionally been proposed an anti-lock system of the three-channel type, for example, which is arranged to secure improved steering performance and running stability of a motor vehicle during a braking operation, while at the same time shortening the braking distance. With such a conventional anti-lock system, signals representing respective wheel speeds Vw1 to Vw4 are provided on the basis of pulse signals derived from speed sensors associated respectively with the wheels; the signals representing the front lefthand wheel speed Vw1 and front righthand wheel speed Vw2 are passed to first and second control logic circuits as first channel speed Vs1 and second channel speed Vs2 respectively; and the lower one of the rear lefthand wheel speed Vw3 and rear righthand wheel speed Vw4 is selected (referred to as "select low" hereinafter) and passed to a third control logic circuit as a third channel speed Vs3. Furthermore, the highest one of the respective wheel speeds Vw1 to Vw4 is selected (referred to as "select high"), and the thus selected highest wheel speed has its acceleration and deceleration limited in the range from +1G to -1G (G is a unit of acceleration equal to the standard acceleration of gravity) to set up a computed vehicle speed Vv, approximate to the actual wheel speed, which in turn is passed to the respective control logic circuits, as disclosed in U.S. Pat. No. 4,741,580. With the respective control logic circuits, a threshold speed Vt is set up which is lower by a predetermined amount .DELTA.V (.DELTA.V=5 km/hour, for example) than and follows the computed vehicle speed Vv (Vt=Vv-.DELTA.V); the aforementioned first to third channel speeds Vs1 to Vs3 are regarded as wheel speed Vw to be controlled (referred to simply as "wheel speed Vw" hereinafter); pressure increasing valves and pressure reducing valves associated with the respective channels are on-off controlled based on the wheel speed Vw, acceleration Vw of the wheel speed, computed vehicle speed Vv, threshold speed Vt and so forth; and in this way, anti-lock control including periodic buildup, reduction and holding of the brake fluid pressure, one cycle of which comprises the time period from a pressure buildup starting point to the next one, is effected to control the wheel speed Vw, as shown in FIG. 1.
With each cycle of anti-lock control as shown in FIG. 1, in a pressure buildup starting point waiting mode, brake fluid pressure buildup is started at a point where the following conditions (1) and (2) are met at the same time:
(1) the wheel speed is higher than the threshold speed Vt; and
(2) it is detected that the wheel speed Vw has reached a high peak or a level close thereto.
In this case, the fact that such a condition that the acceleration or deceleration Vw of the wheel speed is lower than a predetermined acceleration or deceleration Go (including the case where Vw&lt;0) persists for a period of time equal to or longer than a predetermined time To, is regarded as the detection of the wheel speed Vw having reached a high peak or a level close thereto as mentioned above in the paragraph (2).
However, such a conventional anti-lock control system is disadvantageous in the below-mentioned points.
With the conventional system, as mentioned above, the highest one, VwH, of the four wheel speeds is selected (select high); in the case where the acceleration or deceleration of the highest wheel speed VwH is within the range from +1G to -1G, for for example, this speed VwH is used as computed vehicle speed Vv; and in the case where the acceleration or deceleration of the highest wheel speed VwH is outside the range from +1G to -1G, for example, a speed having an acceleration of +1G or a deceleration of -1G is set up as the computed vehicle speed Vv. Thus, in case the speeds of four wheels running on a road surface of a low friction coefficient are dropped so that all the wheels tend to be locked at the same time, the computed vehicle speed Vv decreases with a deceleration gradient of -1G that is one of the follow-up limits therefor, and the threshold speed Vt is lower by .DELTA.V than and follows the computed vehicle speed Vv. In this way, it happens that the wheel speed Vw, even when it has not yet recovered sufficiently and is in a recovering process, goes above the threshold speed Vt so that a high peak of the wheel speed Vw occurs, and thereupon the aforementioned pressure buildup starting conditions (1) and (2) are both satisfied at the same time so that brake pressure buildup is started, as a result of which the wheel speed Vw is further dropped, and early-locking of the wheels tend to be caused.
Especially with a direct-coupled 4-wheel drive system, if, in the course of one wheel speed being recovered after a reduction of the brake fluid pressure, the other wheel speed is increased abruptly, then due to interference between the wheels, said one wheel speed is decreased temporarily; and thereafter, when said one wheel speed is changed from decrease to increase, said other wheel speed is decreased temporarily. In this way, the wheel speed Vw moves toward recovery while changing slightly due to such interference as mentioned above. With a front wheel drive system, the speed of the front wheel when the gear is engaged, moves toward recovery while changing slightly under the influence of engine brake. Especially with a motor vehicle of the direct-coupled 4-wheel drive or front wheel drive type, even when the brake fluid pressure is being reduced or held, it often happens that temporary increase and decrease in the wheel speed Vw occur repetitively while the wheel speed is on the way toward recovery, so that a high peak of the wheel speed Vw is more likely to be detected. Thus, the aforementioned drawback turns out to be more serious.